This invention relates to novel multimodal polyethylenes, polyethylene extrusion compositions comprising low density polyethylene and the multimodal polyethylene, and also to extruded articles made from the polyethylene extrusion compositions.
Low density polyethylene (LDPE) made by high-pressure polymerization of ethylene with free-radical initiators as well as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and ultra low density polyethylene (ULDPE) made by the copolymerization of ethylene α-olefins with Ziegler-Natta and/or single site metallocene catalysts at low to medium pressures have been used, for example: (i) to extrusion coat substrates such as paper board, paper, and/or polymeric substrates; (ii) to prepare extrusion cast film for applications such as disposable diapers and food packaging; and (3) to prepare extrusion profiles such as wire and cable jacketing. Hereinafter, traditional HDPE, LLDPE and ULDPE resins, comprising linear and substantially linear polyethylene resins, are collectively referred to as linear polyethylene. Although LDPE generally exhibits excellent extrusion processability and high extrusion drawdown rates, LDPE extrusion compositions lack sufficient abuse resistance and toughness for many applications.
The density limitations of LDPE resins, approximately 0.915-0.935 g/cc, hinders their use unblended when lower heat seal characteristics are needed, or for higher density applications, such as release paper coating, photographic paper coating where higher modulus is needed. For extrusion coating and extrusion casting purposes, efforts to improve properties by providing LDPE compositions having high molecular weights (i.e., having melt index, I2, less than about 2 g/10 min) are not effective since such compositions inevitably have too much melt strength to be successfully drawn down at high line speeds. While ethylene copolymers with functionalized olefins, such as vinyl acetate, offer lower heat seal temperatures, the chemical properties of such resins make them unsuitable for many uses. No known method to prepare a LDPE with density above about 0.935 has been disclosed. Thus applications requiring such higher densities rely on linear resins, usually blended with LDPE to improve the coating performance, but usually with sacrifice of the desired physical properties.
While HDPE, LLDPE and ULDPE extrusion compositions offer improved abuse resistance, toughness properties and barrier resistance (against, for example, moisture and grease permeation), these linear ethylene polymers cannot be extruded or drawn down at high take-off rates and they are known to exhibit relatively poor extrusion processability in the form of high neck-in, draw resonance and high motor load.
The ultimate extrusion drawdown rate of ethylene α-olefin interpolymers is limited (at otherwise practical extrusion line speeds) by the onset of a melt flow instability phenomena known as draw resonance rather than being limited by melt tension breaks due to “strain hardening” which occurs at higher line speeds and is typical for LDPE and other highly branched high pressure ethylene polymers such as, for example, ethylene-acrylic acid (EAA) copolymers and ethylene vinyl acetate (EVA) copolymers, herein referred to as functionalized LDPE resins.
Linear low density polyethylene (LLDPE) is typically a copolymer of ethylene and an α-olefin of 3 to 12 carbon atoms, preferably 4 to 8 carbon atoms (for example, 1-butene, 1-octene, etc.), that has sufficient α-olefin content to reduce the density of the copolymer to a density of from 0.915 to 0.935 g/cc, the density range available for LDPE. LLDPE resins exhibit improved performance over LDPE in many areas, including improved abuse resistance, toughness properties, sealant properties, range of modulus, barrier resistance (against, for example, moisture and grease permeation). However, in general, linear ethylene polymers exhibit unacceptably high neck-in and draw resonance resulting in relatively poor extrusion processability compared to pure LDPE. Consequently, LLDPE resins are generally considered unacceptable in the extrusion coating industry and are blended with LDPE in commercial applications to improve processability while benefiting from the superior range of physical properties of LLDPE. However, addition of LDPE resins does have some negative impact on the performance properties of LLDPE.
Several compositions containing LDPE blended with linear polyethylene resins have been disclosed. For example, U.S. Pat. No. 5,582,923 discloses compositions with 5% to 20% LDPE, I2<6 g/10 minutes, and linear density 0.85-0.94. Similarly, U.S. Pat. No. 5,773,155 and EP 0792318 disclose substantially linear polyethylene blended with up to 25% LDPE. WO 2005/023912 discloses an extrusion composition containing a minimum of 10% LDPE wherein the substantially linear polyethylene component has a melt index >20 g/10 min. The compositions disclosed in these references can be blended as part of the powder pelletization stage in the gas phase process for manufacturing linear polyethylene resins. However, not all manufacturing installations have such process capability. In general, these compositions cannot be prepared in the solution process for manufacturing linear polyethylene resins prior to pelletization because the capability to feed the required amounts of LDPE is not available or would require unacceptable reduction in reactor rate. Thus, these blends must be made subsequent to pelletization at substantial cost, (e.g., costs related to reheating the polymers and transportation).
Often solution process linear polyethylene plants are designed with the capability to side-arm a quantity of material into the molten polymer flow prior to pelletization. The maximum quantity that may be added by side-arm addition is generally less than 20% of the polymer flow and more often <6% of the polymer flow. This side arm addition capability is generally utilized for addition of various additives to the polymer such as anti-oxidants, slip agents and the like.
Thus, there is a need for linear polyethylene compositions of a wide range of densities, which when blended with LDPE resin, exhibit acceptable coating behavior and wherein the blend comprises <20% LDPE of the total resin weight and preferably <6% LDPE.
Another need arises from the limited availability of autoclave LDPE. Although autoclave LDPE is generally preferred for extrusion coating processes, there is much wider availability of tubular LDPE compared to autoclave LDPE. Tubular LDPE, however, tends to cause the formation of smoke during the extrusion coating process. Moreover, when blended with linear polyethylene, greater amounts of tubular LDPE, compared to autoclave LDPE, are required to attain acceptable processability, e.g., low neck-in and high drawdown rates. The amount of tubular LDPE generally needed to obtain acceptable extrusion processability is at least 25% of the total resin when blended with known linear polyethylene resins. Such large amounts of tubular LDPE cause substantial smoking during extrusion and are often associated with wax build-up on various parts of the extrusion equipment, such as rollers, resulting in undesirable equipment shut-down. To take advantage of the wider availability of tubular polyethylene, it would be desirable to have a linear polyethylene composition that, when blended with less that 25% tubular LDPE by weight of the total resin, exhibits acceptable extrusion processability.
As described hereinafter, the present invention substantially fills the need for ethylene polymer extrusion compositions having high line speeds, high resistance to draw resonance and substantially reduced neck-in, comprising <20% autoclave LDPE of the total resin weight and preferably <6% autoclave LDPE, and a method of making such compositions. Embodiments of the invention further fills the need for ethylene polymer extrusion compositions yielding acceptable extrusion coater performance comprising tubular LDPE comprising 15% to 20% of the total resin composition. The linear polyethylene resin component of embodiments of the present invention comprise a high molecular weight component having substantial long chain branching and a low molecular weight component and is referred to hereinafter as Multimodal polyethylene or Multimodal PE. The compositions of the present invention can be used in conjunction with known resin manufacturing and extrusion coating equipment and equipment modifications and the combined or synergistic benefits of the present invention and known solutions can also be realized.